This application is a continuation-in-part of application Ser. No. 560,129, filed Dec. 12, 1983, now abandoned.
This invention relates to a ferritic steel having improved cyclic oxidation resistance and creep strength at elevated temperature. More particularly, in the form of cold rolled strip, sheet, bar, rod and wire which has been subjected to a final anneal at 1850.degree. to 2050.degree. F. (1010.degree. to 1120.degree. C.), a preferred steel of the invention having a ferritic microstructure exhibits the above properties by reason of purposeful addition of silicon, a carbide and nitride former, and columbium within critical limits. Control of aluminum to a low value confers excellent weldability and formability without sacrifice of other properties. A synergistic improvement in creep strength and improved cyclic oxidation resistance at elevated temperature results from the combination of a silicon addition within the broad range of 0.8% to 2.25%, addition of sufficient carbide and nitride former to combine with substantially all the carbon and the nitrogen, addition of a small amount of columbium substantially all of which will be uncombined as a result of the carbide and nitride former addition, and a final high temperature anneal. The combination of properties is achieved throughout a wide range of chromium levels, viz. from about 1% to about 25%, but a fully ferritic microstructure may not be obtained at chromium plus molybdenum levels less than about 8%.
The automotive industry is a large user of flat rolled ferritic stainless steels for engine exhaust components. A standard stainless steel for this purpose has a nominal composition of about 0.03% maximum carbon, about 0.25% manganese, residual phosphorus and sulfur, about 0.5% silicon, about 12% chromium, about 0.2% nickel, about 0.4% titanium, about 0.1% maximum aluminum, about 0.02% maximum nitrogen, and balance essentially iron.
The present invention provides a substitute for the above stainless steel, having improved properties, not only for automotive exhaust components, but also for powder metal articles and welded articles.
A steel having substantially improved elevated temperature strength and oxidation resistance, in comparison to the above standard steel, is disclosed in U.S. Pat. No. 4,261,739. In broad ranges the steel of this patent consists essentially of, in weight percent, from about 0.01% to 0.06% carbon, about 1% maximum manganese, about 2% maximum silicon, about 1% to about 20% chromium, about 0.5% maximum nickel, about 0.5% to about 2% aluminum, about 0.01% to 0.05% nitrogen, 1.0% maximum titanium, with a minimum titanium content of 4 times the percent carbon plus 3.5 times the percent nitrogen, about 0.1% to 1.0% columbium, with the sum total of titanium plus columbium not exceeding about 1.2%, and remainder essentially iron. A preferred steel in accordance with this patent has a nominal composition of about 0.02% carbon, about 0.25% manganese, about 0.02% phosphorus, about 0.005% sulfur, about 0.5% silicon, about 12.0% chromium, about 0.20% nickel, about 0.02% nitrogen, about 0.3% titanium, about 0.6% columbium, about 1.2% aluminum, and balance essentially iron. Such a preferred steel exhibits optimum elevated temperature strength and oxidation resistance in the cold rolled form when it is subjected to a final anneal at 1850.degree. to 2050.degree. F.
While this patent recognizes that aluminum in excess of about 1% can affect weldability adversely, relatively high aluminum levels, with a minimum of about 0.75%, must nevertheless be present in order to obtain the excellent elevated temperature oxidation resistance of this steel. Accordingly, the likelihood of poor weldability under some types of welding operations is present in the steel of this patent.
At column 8, lines 25-43 of U.S. Pat. No. 4,261,739 it is alleged that variations in aluminum content (between 0.77% and 1.33%) do not markedly affect sag resistance, and hence aluminum can be added at a level low enough to improve weldability. On the other hand, an increased aluminum content improves cyclic oxidation resistance. For an optimum balance of propertiers it is concluded that aluminum should be between about 1.0% and 1.5%.
Olsen Cup tests of welded sections, as a determination of formability, can exhibit considereable scatter in values due to the effects of sample thickness, welding speed, heating conditions, shielding gas and welding method. Thus, in U.S. Pat. No. 4,261,739, Olsen values for weldments, reported in Tables VII and X show little correlation between aluminum content and cup height. An aluminum content of 0.77% exhibited formability inferior to aluminum contents of 1.24%, 1.27% and 1.18% (Heats I, J, L and M in Table X), although superior to a 1.7% aluminum content (Table VII).
In U.S. Pat. No. 4,261,739, the titanium content is increased to compensate for a decreased aluminum content (Column 7, lines 54-57). However, silicon is maintained relatively constant within a range of about 0.45% to 0.6%. In every example of steels of the invention, aluminum is substantially higher than silicon.
In contrast to the above disclosures, the present invention constitutes a discovery that silicon can be substituted at least partially for aluminum and also partially for chromium, with a consequent improvement in weldability while at the same time retaining excellent oxidation resistance and creep strength at elevated temperature.
An article entitled "Influence of Columbium on the 870.degree. C. Creep Properties of 18% Chromium Ferritic Stainless Steels" by J. N. Johnson, SAE Technical Paper Series, 810035, February 1981, reports tests on an 18% chromium steel containing molybdenum, titanium and columbium. In the test samples, silicon ranged from 0.08% to 0.74%, and uncombined columbium ranged from 0.11% to 0.58%. It was concluded on the basis of reported tests that a significant improvement in 870.degree. C. creep strength of 18% chromium steels was obtained with the combination of about 0.5% free (uncombined) columbium and a high final annealing temperature at 925.degree. to 1150.degree. C. (about 1700.degree. to about 2100.degree. F.). In these test samples, aluminum was absent except in one sample which contained 1.89% aluminum, 0.71% silicon, 0.35% titanium and no columbium. This article contains no discussion regarding the effect of silicon or aluminum, other than reference to a Laves phase which, although primarily intermetallic compounds of iron-molybdenum or iron-columbium, may contain substitutional elements such as chromium, manganese and silicon.
"Effect of Molybdenum on Creep Properties of a Ferritic 18Cr-Nb-Ti Steel for Catalytic Converters", J. D. Redmond et al, Journal of Metals, Feb. 19, 1981, pages 19-25 reports the effect of molybdenum and columbium on creep-rupture properties of an 18% chromium steel. It is concluded that an additional strengthening mechanism in molybdenum-containing steels may result from the change in composition of the Laves phase where columbium decreases with increasing molybdenum contents. The displaced columbium is then available for further dispersion strengthening by carbide precipitation.
Ferritic, chromium-containing steels containing one or more of aluminum, titanium, columbium, silicon or zirconium are disclosed in U.S. Pat. Nos. 3,909,250; 3,782,925 and 3,759,705, and British Pat. No. 1,262,588. These alloys, while exhibiting improved oxidation resistance at elevated temperature, nevertheless have poor creep strength at elevated temperature and possible weldability problems.
Japanese Pat. No. 20,318 (published in 1977) and Japanese Pat. No. 107,761 (published in 1980) disclose ferritic alloys containing titanium and columbium, and tantalum, hafnium or tantalum plus zirconium, respectively. Neither suggests the presence of uncombined columbium in combination with silicon at a level greater than 1.0%.
NASA TN-D No. 7966 published in 1975, discloses modifications in 15% and 18% chromium ferritic steels wherein it was concluded that addition of 0.45% to 1.25% tantalum to a nominal 18% chromium, 2% aluminum, 1% silicon and 0.5% titanium steel provided the greatest improvement in fabricability, tensile strength and stress-to-rupture strength at 1800.degree. F., along with oxidation resistance and corrosion resistance at elevated temperature. After cold rolling to final thickness, a final anneal at 1000.degree. C. was conducted in the processing of these test alloys.
An article by H. E. Evans et al, in Oxidation of Metals, Vol. 19, Nos. 1/2, 1983, pages 1-18, describes the influence of silicon on the oxidation resistance of nitrided austenitic stainless steels of nominal 2% chromium-25% nickel composition. A series of such steels, also containing froim 0.005% to 0.050% carbon, 0.42% to 0.74% manganese, 1.44% to 1.56% titanium, and 0.05% to 0.21% columbium, was prepared with silicon levels ranging from 0.05% to 2.35%. Cold rolled strips were nitrided at 1423.degree. K. (2102.degree. F.) and tested for oxidation resistance at 1123.degree. K. (1562.degree. F.). It was found that chromium-rich oxide surface films developed in all cases, and the film thickness increased parabolically with time. The parabolic rate constant was at a minimum at 0.92% silicon. The reason for failure of higher silicon levels (about 1.5% to 2.35%) to improve oxidation resistance was postulated as being perhaps due to removal of silicon from solution by precipitation.